Scanning probe methods are being used to study an increasing range of surface phenomena. These surface phenomena include surface topography, dielectric and magnetic properties, molecular manipulation and many other effects that occur on the micrometer to the subnanometer scale. Scanning probe microscopes include scanning tunneling microscopes (STMs) and atomic force microscopes (AFMs). Both STMs and AFMs utilize a sharp probe tip to interact with the target surface to generate a topographic surface scan. The parameters used for image formation are different for STMs and AFMs. In an STM system, the modulations of the tunneling current are used to determine surface topography while in an AFM system, the varying force between the probe tip and the target surface is used to determine the surface topography.
STM systems may also be employed to monitor the tunneling current versus the bias voltage at fixed locations on the target surface. The STM probe tip is positioned at a particular location above the target surface. A spectrum is produced by sweeping the bias voltage and measuring the tunneling current. This provides information regarding the density of surface states in the target surface proximate to the STM probe tip.
The use of STM and AFM systems has enabled relatively routine imaging of surfaces at nanoscale resolutions, but more information may be desired about the surface. A desirable measurement modality is one that allows surface imaging of unaltered samples while at the same time providing a means for characterizing a physical property associated with the localized target surface.